Mechanical sandwich panel spacer with improved torque resistant anchorage



Jan. 10, 1967 F. w. ROHE ETAL 3,296,765

MECHANICAL SANDWICH PANEL SPACER WITH IMPROVED TORQUE RESISTANT ANCHORAGE Filed June 29, 1964 2 Sheets-Sheet 1 I611. I512 J 7 f 2; 297 F L 515 k 1 7 zg 3 9 5 2' M 4 -23 17 '55 7- 19 15 1 -24 172505901. W 130 42 {fl/m4 E$ gpflE 4M,

INVENTORS JrrORMe;

Jan. 10, 1967 F, w. ROHE ETAL 3,296,765

MECHANICAL SANDWICH PANEL SPACER WITH IMPROVED TORQUE RESISTANT ANCHORAGE Filed June 29, 1964 2 Sheets-Sheet 2 74 f 157 I 72 as a; 5

III 1 2/ M United States Patent 3,296,765 MECHANICAL SANDWICH PANEL SPACER WITH IMPROVED TORQUE RESISTANT ANCHORAGE Frederick W. Roh, 5191 S. Bradford, Placeutia, Calif,

and Charles S. Phelan, Tustin, Calif; said Phelan assignor to said Roh Filed June 29, 1964, Ser. No. 378,714 11 Claims. (Cl. 52-617) This application is a continuation-in-part of our pending application Serial No. 248,969, filed January 2, 1963, for Sandwich Panel Spacer With Torque-Resistant Mechanical Anchorage, now Patent No. 3,197,854.

This invention relates to fasteners for use in lightweight sandwich panels such as are commonly employed for interior wall, fioor and ceiling structure of airplane cabins etc. The general object of the invention is to provide a fastener of the insert or spacer type having improved means for mechanical anchorage thereof to one or both skin sheets of the sandwich panel structure, and avoiding the necessity for utilizing a potting compound or molding resin for anchoring the fastener in the sandwich structure.

An important object of the invention is to provide a fastener having improved means for anchoring it against rotation in a sandwich panel when a thread stud or bolt is being attached thereto. A common means for attaining such torque-anchorage is to fill the hole in the panel around the insert or spacer with a potting resin in a liquid or plastic state (e.g. an epoxy resin). Such a potting resin has some undesirable aspects such as (1) the material is expensive, and (2) in the higher temperature ranges, it softens, loses much of its holding strength, and deteriorates. It is therefore desirable to avoid its use if possible, especially when it will be subjected to high temperatures. At the same time, the fastener must be anchored to the sandwich panel securely enough to resist the torque imposed upon it when a fastener stud is threaded into it or removed, and in addition, the fastener must resist axial pull applied by the coupled part, tending to pull the fastener out of the panel. Furthermore, in the case of a two-part fastener of the spacer type, the components must be securely anchored against pulling apart.

Attempts have hitherto been made to provide spacertype fasteners having multiple parts providing mechanical (clamping) anchorage to sandwich panel skin sheets, the parts being coupled to one another by an interference or friction fit between the parts. This type of coupling in spacers is proving to be unsatisfactory to meet the present day requirements for security of coupling.

We have found that with some spacer-type fasteners, it is possible for the fastener to fail at one end, releasing its grip on the one skin sheet, while remaining attached at its other end, and in thus failing, to gouge out a fairly large section of the flimsy core structure between the sheets.

With the foregoing problems in mind, the invention aims to provide a fastener having means to anchor it to one or both skin sheets of a sandwich panel solely by mechanical engagement of the skin sheet or sheets; which will provide adequate torque-anchorage through such mechanical engagement of the skin sheet or sheets; which will maintain secure anchorage in high temperatures such as are imposed on missile structures; wherein both ends can be anchored to respective skin sheets with sufficient grip to actually tear chunks of both skin sheets out of the panel before the grip of either skin sheet will be released; and wherein gripping engagement of the ends of the fastener with both skin sheets can be developed in successive stages of unitary fastener-setting operation.

3,296,765 Patented Jan. 10, 1967 ICC Another object is to provide such a fastener having a collapsible section adapted to collapse outwardly during installation of the fastener in a sandwich panel in order to attain the following desirable results: (a) development of a circumferential fold providing anchorage of an end of the fastener to a skin sheet; and (b) adjustment of the length of the fastener to compensate for variations in core depth and skin sheet thickness of the sandwich panel.

Another object is to provide a fastener having means for self-dimpling of a head portion thereof into the material of a sandwich panel skin sheet surrounding an opening therein, and for bringing another part of the fastener into mechanical interlocking engagement with the inner face of the dimpled ski sheet to clamp the same against the rear face of the head. Thus, the invention avoids the necessity for pre-dimpling of either skin sheet.

More specifically, the invention contemplates a twopart spacer-type fastener comprising a plug and a spacer sleeve which are cooperative, during installation, to effect clamping of one skin sheet between one end of the sleeve and an opposed head on the corresponding end of the plug; and to establish positive mechanical interlock between the other skin sheet and the other end of the sleeve by upsetting and outward bulging of a weakened section of the sleeve adjacent the head on said other end of the sleeve.

A further object is to provide a spacer-type fastener which, in addition to the characteristics outlined in the previous paragraph, is provided with means for mechanically locking the plug to the spacer sleeve to maintain the clamping engagement between the head of the plug, the opposed end of the sleeve and the interposed skin sheet.

A further object is to provide a spacer-type fastener in the installation of which the clamping of the one skin sheet between the plug head and sleeve, the upsetting of the other end of the sleeve into locking engagement with the other skin sheet, and finally, the locking of the plug to the sleeve, occur as consecutive stages of a continuous installation operation in which axial compression is applied to the fastener.

Yet another object is to provide such a spacer-type fastener wherein self-dimpling of both skin sheets by the respective ends of the fastener will occur during the continuous progressive setting operation referred to above, in a manner such that, by utilizing countersunk type heads, the respective ends of the spacer can be brought into flush relation to the outer surfaces of the panel.

A further object is to provide a spacer-type fastener having means for anchoring the respective ends thereof to the respective skin sheets of a sandwich panel so securely as to prevent either end of the fastener from becoming released from its respective skin sheet (i.e. so that failure can occur only by tearing away respective fragments of both skin sheets).

Another object is to provide such a spacer-type fastener wherein the spacer part has at one end a collapsible section which incorporates a short column characteristic such as to insure (l) uniform coaxial collapse and bulging as contrasted to long column bending (2) deformation resistance of sufficiently high value to transmit compressive forces adequate to effect completion of a dimpling and clamping of the opposite skin sheet between the other end of the spacer and the head of the cooperating plug part, with torque-anchoring coining of radial flutes on the shoulder of such head into said opposite skin sheet; and (3) with a filleted cross-section such as to cr d the fold of the collapsing section toward the adjacent skin sheet so as to attain adequate clamping of such adjacent skin sheet between the fold and the opposed head of the spacer.

In the setting of collapsible mechanical spacers we I have discovered that a problem is involved in maintaining sufficient column strength in the collapsible wall section of the spacer to complete the forming of a dimple between a counterbored end of the sleeve and a correspondingly frust-conical shoulder on the head of the plug, before the weakened wall section commences to collapse. As a solution of this problem, the invention embodies an improved feature wherein column-strength of the collapsible section of the sleeve is increased with reference to its wall thickness, by externally knurling the collapsible wall section with a fine knurling of a fluted type parallel to the sleeve axis.

Accordingly, a further object of the invention is to provide an improved ratio of compression strength to wall thickness of a collapsible spacer section.

Based on the foregoing, the invention attains the following improved advantageous results:

( 1) Provides complete structural continuity.

(2) Develops full load capability of sandwich structure.

(3) Provides torque-out capacity greater than the strength of its bolt or stud.

(4) Actually increases the strength of the sandwich panel at the fastener area.

(5) Eliminates crippling of the core during dimpling by providing columnar support of the area through the spacer component.

(6) Provides a two-part mechanical spacer of a compression-setting type adapted for two-stage clamping of its respective ends against respective skin sheets of a sandwich panel, with an improved ratio of compression strength to wall thickness in a collapsible section of a sleeve part thereof, and an improved setting action in which the first stage of clamping action, at one end of the fastener, is developed to a satisfactorily complete condition before the weakened other end of the sleve collapses in the second stage of clamping action.

Another object of the invention is to provide an improved method of installing an insert or spacer in a lightweight sandwich panel, and one which can be carried out by optional use of several different types of tooling (either power press or portable squeeze tooling of hydraulic, pneumatic or screw jack type).

Still a further object is to provide an improved tool for installation of the fasteners contemplated by the invention.

Another object is to provide a fastener for improved mechanical anchorage in a lightweight sandwich panel, which fastener can be made available either in the form of an insert or a spacer, either with or without internal threads for anchoring a threaded stud or bolt, and in any of the various selective head styles.

A further object is to provide such a fastener which can be adapted to accumulated tolerances of sandwich dimensions, with variations of as much as .020 inches above or below a specified dimension.

A further object is to provide such a fastener which can be installed from either side of a sandwich panel.

While the invention is especially useful in spacers of aluminum alloy, it is also applicable to steel spacers.

Other objects and advantages will become apparent in the ensuing specification and appended drawing in which:

FIG. 1 is a combined axial sectional-elevational view of a fastener of a spacer type, including a plug with threaded bore for attachment of a threaded stud, shown in an initial stage of installation in a lightweight sandwich panel;

FIGS. 2, 2a and 2b are combined elevational-axial sectional views of the same, shown in succeeding stages of installation;

FIG. 3 is a combined sectional-elevational view of the same, fully installed in a sandwich panel of intermediate thickness within the range in which the fastener is applica-ble;

FIG. 4 is a sectional-elevational view of a fastener of the spacer type, installed in a sandwich panel of minimum thickness within the range to which the fastener is applicable, and embodying a modified form of the invention wherein the plug has an unthreaded bore;

FIG. 5 is an axial sectional view of a spacer-type fastener embodying another modified form of the invention wherein the plug has a closed head and a blind thread;

FIG. 6 is a view, partially in axial section and partially in elevation, of the plug member of a spacer type fastener embodying a modified form of head which can be utilized in the plug member of any of the several forms referred to above;

FIG. 6a is a side view, partially in axial section of a spacer embodying a collapsible wall section of improved column strength;

FIG. 7 is a partially sectional, partially elevational view of a blind fastener of the spacer type, embodying another modified form of head which can be utilized in any of the several forms of the invention referred to above; and installed in a sandwich panel of maximum thickness within the range to which the fastener is applicable;

FIG. 8 is a combined sectional-elevational view-of a blind-type fastener (for blind installation in a sandwich panel with anchorage to only one skin sheet) embodying the important torque-anchor feature of the invention;

FIG. 9 is an elevational view, partially in axial section, of a screw-jack installation tool embodying the tool feature of the invention and illustrating one embodiment of our improved installation method (as applied to the fastener of FIG. 8);

FIG. 10 is a fragmentary view of the installation tool of FIG. 9, associated with the fastener of FIG. 8 which is shown in fully installed condition in a sandwich panel, partly in elevation and partly in section; and

FIG. 11 is a fragmentary sectional view of a modified form of the tool of FIG. 9, adapted to the installation of the spacer type fastener of FIG. 3.

DESCRIPTION Referring now to the drawings in detail, and FIG. 3 in particular, we have shown therein, as an example of one form of our improved fastener installed in a lightweight sandwich panel A, a spacer-type fastener comprising a spacer sleeve B and a plug C locked therein, both ends of the spacer assembly being locked to the panel A.

Sandwich panel A comprises front and back skin sheets 15 and 16, which may be of lightweight sheet metal such as aluminum alloy, and a low-density core 17 which may be of honeycomb cell structure, with the cell walls (indicated schematically at 18) extending transversely between the skin sheets 15 and 16 and suitably secured thereto at their end edges. The fastener plug C and spacer sleeve B are inserted through an opening 19 in skin sheet 15 and a larger opening 20 in skin sheet 16 respectively. Around opening 19, skin sheet 15 has an inwardly depressed dimple 21 which is clamped between opposed end portions of fastener parts B and C. Around opening 20, skin sheet 16 has an inwardly depressed dimple 22 which is clamped between adjacent, axially-opposed end portions of spacer sleeve B at its opposite end.

Spacer sleeve B (FIG. 1) comprises a relatively thickwalled sleeve body 23 for a majority of its length, a frustoconical counterbore 24 at its innner end, mating with the inner face of dimple 21, a head 25 at its outer end, and an outwardly folded annular fold 26 (FIG. 3) providing a secondary head in opposed, adjacent relation to end head 25, formed by collapsing a weakened section 45 of the sleeve wall, the dimple 22 being mechanically locked between the heads 25 and 26. Sleeve body 23 has a through bore 49, of a diameter to freely but snugly receive plug C.

Plug C comprises a tubular neck 27 telescoped into the main sleeve body 23 of spacer B, an integral head 28 at its outer end, and an integral, expanded lip 29 at its inner end. Head 28, in this form of the invention, is of flush type, having, as its inner face, a frusto-conical shoulder defined by a plurality of raying fiutes 39 which are indented into the outer face of dimple 21 and thereby torque-anchored thereto (locking the plug C against rotation in the panelA).

The retainer lip 29 is swaged tightly against a frustoconical infernal shoulder 31 which defines the inner end of the thickened sleeve body 23. The auxiliary head 26 comprises a first fold 32, the inner face of which is substantially of the same inclination and a flush continuation of the shoulder 31, and a second fold 33 which is reentrant and is engaged against the inner side of skin sheet dimple 22. Fold 33 is joined to fold 32 by a peripheral bend 34 and to head 25 by an inner reverse bend 35, the series of folds and bends being of S-section convolution.

The dimple 21 of skin sheet is tightly clamped between head 28 of plug C and counterbored end 24 of spac er B. The dimple 22 is mechanically locked between the outer end head 25 and the auxiliary head 26 of spacer B. In some installations, the dimple 22 may be tightly or snugly clamped between the heads as indicated in FIG. 4 (e.g. where the aggregate thickness of panel A including core depth and skin sheet thickness, is less than a predetermined median value for a selected range); whereas in other cases (e.g. where the aggregate panel thickness is greater than the median value) the con'vo-l'utions 32-35 may not be completely closed, the fold 33 may be disposed short of full mating contact with the inner face of dimple 22, but the inner edge of dimple 22 at the margin of opening 26 will in any event be tightly engaged in the inner bend 35 of the convolutions. FIG. 3 discloses an approximately average condition of folding of secondary head 26.

Because of the tight engagement of the inner edge of dimple 22 in lbend 35, and the full seating engagement of the outer face of the dimple against head 25', the spacer of this installation is securely anchor-ed to skin sheet lid notwithstanding the fact that its collapsed section is not fully closed.

In the particular fastener illustrated in FIG. 3, both of the heads 25 and 28 are of the flush or countersunk type, and the neck 27 of plug C has a threaded bore 36 for reception of the threaded shank of a fastener stud, bolt or the like. Also, the fastener assembly is of a through type, with both ends open so that a stud or bolt can be inserted from either end. The plug C, being torqueanchored to skin sheet 15, will resist the turning torque of a stud or bolt being screwed into or unscrewed from the plug. The torqueanchorage is assisted by the locked connection between the outer end of spacer B and skin sheet 16, plus the locked connection between plug C and spacer B. Where a threaded bore is utilized, a suitable thread lock is also incorporated. Merely by way of example, we have shown in FIG. 1 an inwardly deformed locking thread section 37 in a weakened annular web section of neck 27, defined at the bottom of an annular external groove 38 therein. Other equivalent thread locks can be used (e.g. of the type embodying a plug of nylon or equivalent material inserted through a radial bore in the wall of neck 27).

Modified forms of the spacer fastener of FIG. 3 may embody variations in the plug C and/ or spacer B as illustrated in FIGS. 4-7, as follows:

FIG. 4 illustrates a fastener assembly embodying a plug C1 having a smooth cylindrical bore 36a instead of the threaded bore 36 of FIG. 3, and also shows a secondary head 26a of a maximum folded condition (completely bottomed) in a panel A1 of minimum thickness within the fastener range, the assembly being otherwise the same.

FIG. 5 illustrates a fastener assembly embodying a plug C2 having a blind threaded bore 361) the bottom of which is defined by a closed head 28b. Thus, only one end of the spacer is open to receive a fastener stud or bolt, although the spacer extends entirely through the panel A and has both ends secured thereto as in FIG. 3.

FIG. 6 illustrates a plug C3 having a flush head 280 provided with a counterbore 44]! at the outer end of bore 36c. The bore 360 may be internally threaded or a smooth cylindrical bore, and in either instance may be either a through bore as illustrated in FIGS. 3 and 4, or a blind bore as illustrated in FIG. 5.

FIG. 7 ilustrates a plug C4 having a flat head 28d provided on its back side with a shoulder defined by radial flutes 30:! disposed in a flat plane. The head 28d does not dimple the skin sheet 15, although it will become embedded therein substantially to the depth of its flutes 30d, to establish the torque-anchorage.

FIG. '7 also shows a sleeve B4 having a flat head 25d which can be electively utilized in lieu of the flush type head 25 of FIG. 3, and illustrates a secondary head 26d of minimum bulge, in a panel A4 of maximum thickness within the fastener range. FIG. 7 may be regarded as somewhat exaggerated as to the open condition of the convo lutions, for all except a few exceptional cases.

Improved column strength, FIG. 6a, is attained by knurling the external surface of collapsible section 45a with shallow, closely spaced linear indentations defining fine ridges 42 extending in closely-spaced relation parallel to the axis of the insert. The ridges 4 2 may have a height, above the bottoms of the indentations, in the approximate range of .00 3.010 inch and the circumferential spacing between ridges may be in the range of approximately .0l0.025 inch, although knurling outside the limits of these ranges may be utilized. We find that the knurling adds an unexpected degree of control over column strength and increase in the degree thereof, such that the Wall thickness of section 45a may be reduced below that of the section 45 of FIG. 1, and better setting of dimple 21 between head 28 and countersink 24, coupled with improved clamping of the other dimple 22 between head 25 and annular fold 26, is attained.

For a spacer of anodized 606lTC aluminum alloy having an external diameter of approximately .575 inch, a counterbore diameter of approximately .486 inch, a wall thickness of approximately .89 inch and a counterbore depth of approximately .28 inch (providing an axial height of weakened wall section 45a or roughly A1 inch) I find that a 53 pitch (53 ridges per inch) and approximately between .003 and .GlO inch height, is satisfactory for the purposes of the invention and will increase the column strength over that of a spacer of the same dimensions and material but with a smooth cylindrical outer surface throughout, to a significant extent (e.g. by a percentage increase of approximately 25% to 50% Furthermore, by utilizing such knurling, the wall thickness stated in the example given may be approximately 50% less than the wall thickness of an un-knurled spacer in which all other dimensions are the same as those given in the example, and at the same time an approximately 35% increase in column strength over that in the un-knurled spacer can be achieved.

INSTALLATION FIG. 1 illustrates the components of the spacer assembly of FIG. 3, as these components exist prior to assembly. The head 25 of spacer B is joined to the main body section 23 thereof by a weakened or thinned neck portion 45 the outer surface of which is a continuation of the outer surface of sleeve body 23, of the same diameter, and the inner wall of which is defined by a counterbore 7 46. The frusto-conical shoulder 31 defines the bottom of this counterbore. Shoulder 31 is joined to counterbore 46 by a fillet 47 having a radius approximately equal to the difference between the radii of counterbore 46 and the through bore 49. The plug C, at its inner end, has a thinned cylindrical tip 29, defined by a counterbore 37.

The spacer B fits loosely in the aperture 20 of skin sheet 16. The neck 27 of plug C fits loosely in the aperture 19 of skin sheet 15. The two components are inserted from opposite sides of the panel A as indicated.

Setting of the fastener is effected by applying opposing forces to the spacer B and plug C respectively, to force them against the respective skin sheets and 16. This can be done, as illustrated in FIG. 2, by supporting the head of plug C against an anvil or bucking member 50 which can be the bed of a press, or a portable locking tool. On the opposite side of the panel A, there is inserted into the open end of the fastener assembly, a setting tool comprising a ram 51 having a radial shoulder 52 for applying pressure to the spacer head 25, a projecting nose 53 having a slightly concave frusto-conical swaging face 54 for engagement with the weakened tip 29' of plug C, an entry cone 55 forwardly of swaging face 54, and a pilot 56 projecting forwardly from cone 55. Pilot 56 is dimensioned for locating fit in the bore 36, 36a, or 36b etc. and prevents radial drift of the tool during the setting operation (which might result in eccentric deformation of tip 29' and weakened section 45).

The setting operation is effected by exerting downward pressure through ram 51 and upward counter-pressure through anvil 50, and progresses in several stages, in the following order:

(1) The annular portion of skin sheet 15 around opening 19 (FIG. 2) is clamped between the head 28 of plug C and counterbore 24 of spacer B and the head 28 is indented into the skin sheet, forming the dimple 21. The flutes 30 are embedded in the dimple 21, establishing the torque-anchorage of plug C to skin sheet 15 which resists turning of the plug in the skin sheet when a fastener stud is threaded thereinto, or when torque is applied from any other source.

(2) When dimple 21 is solidly bottomed between countersink 24 (and head 28 in turn solidly bottomed against anvil 50) the weakened section 45 of spacer B commences to collapse under the pressure applied by ram 51, as shown in FIG. 2a. The nose portion 53 of the ram supports the inner wall of weakened section 45 against inward collapse and thus the collapsing is in the outward direction. Since the head 25 is being driven inwardly by ram 51, the outward bulge of weakened section 45 occurs on the inner side of skin sheet 16, approximately as indicated by the arrow in FIG. 2a while the head 25 approaches the outer face of the skin sheet. Thus, the margin of skin sheet 16, at aperture 20, becomes caught between the overhanging head 25 and the bulging section 45' of the spacer.

As the operation progresses, the margin of skin sheet 16 is engaged by head 25 and indented inwardly (FIG. 2b). It then makes engagement with the outward shoulder of the bulging section 45 and commences to apply inward pressure thereto, thus assisting the further collapsing of the section 45'. At its periphery, the collapsing section 45' offers resistance to the inward indenting of skin sheet 16. At the end of the operation, there is developed in the skin sheet 16, around aperture 20, the dimple 22 substantially mating with the inward conical shoulder of head 25, and the auxilary head 26, 26a or 26d, depending on the thickness of the panel A. The large radius of fillet 47 provides a buttressing characteristic in the base portion of neck 45, which directs the developing fold 45' (FIG. 2a) toward skin sheet 16 a indicated by the arrow in FIG. 2b. The buttressing effect is derived from the graduation in thickness from a maximum at shoulder 31 to a minimum where fillet 47 becomes tangent to the cylindrical wall of counterbore 46. We find that the buttressing feature promotes the crimping of secondary head 26 against the margin of skin sheet 16 are aperture 20 to provide adequate clamping engagement of the skin sheet. In attempts to utilize a cross-section wherein counterbore 46 was joined directly to an abrupt offset shoulder (at the location of shoulder 31) in a sharp internal corner, we found that the collapsing wall section 45' tended to yield away from skin sheet 16, failing to establish clamping engagement therewith. This difficulty was overcome by the buttressing effect of fillet 47.

(3) During the final stages of the collapsing operation described above, at a point between the condition shown in FIG. 2a and the fully set condition (e.g. as shown in FIG. 3), the swaging surface 54 of the tool engages lip 29 of the plug C (FIG. 2b) and swages it against the frustoconical shoulder 31 of sleeve B (FIG. 3) to lock the plug and sleeve together, This operation continues during the final stage of dimpling the head 25 into the skin sheet 16 and forming the auxiliary head 26, and both operations are completed simultaneously. At the completion, lip 29' is snugly engaged against spacer sleeve shoulder 31 and the entire assembly is locked together with the respective skin sheets clamped in the respective ends of the fastener.

Where the flat heads 25d and 252, or either of them, are employed, since dimpling does not take place, the collapsing stage will be of lesser extent, and the open type auxiliary head 26d seen in FIG. 7 may be formed. Also, where the flat head 28:! is employed, the main body 23d of spacer B4 will normally have a squared or radial end face as shown in FIG. 7, for compressing the skin sheet 15 against the radial flutes 30d of head 28d.

An important feature of the invention is the absorption of tolerances in panel thickness, in the bulge of secondary head 26, 26a, 26d between maximum and minimum collapse. The weakened section 45 has a column length sufficient to provide an adequate amount of bulging to meet the tolerance requirements, yet it has a short column characteristic which insures true symmetrical collapse as contrasted to a long column characteristic (failing by bending when subjected to compression). The buttressing effect assists in attaining the short column characteristic, in that a very substantial portion of the length of weakened section 45 is of more than the minimum thickness.

The short column feature is further characterized by having strength to remain unyielding until the head 28 of plug C has become fully seated in or against skin sheet 15, with its ilutes 30 fully embedded therein.

The full blind insert assembly of FIG. 10 comprises a single fastener part D mounted in a sandwich panel A in a blind hole 60 extending from an aperture in the one skin sheet 15 and having a closed bottom defined by the other skin sheet 16. The insert D has a head 28 provided with a fluted shoulder 30 the flutes of which are embedded in the skin sheet dimple 21 and locked thereto by a bulged auxiliary head 61 formed in a weakened section of the insert. This weakened section, prior to bulging, is shown at 62 in FIG. 8. A shallow annular groove 63 is provided therein at the inner extremity of head 28. Beyond the weakened section 62, the insert has a relatively thick main body section 27 provided with an internal thread 36 to receive a threaded fastener stud. Since the insert is a fully blind type, it will normally have a closed bottom 64.

The auxiliary head 61 engages the margin of dimple 21 to lock the flutes 30 in an embedded engagement with the dimple 21, thus maintaining the torque-anchorage attachment of the insert to the skin sheet 15.

Tooling for installation is shown in FIG. 9. The tool comprises an anvil 70 having a head 71 for pressure engagement against the insert head 28, a flat-sided shank 72 for holding engagement by a suitable wrench, and having a suitable nut 73 provided with an internal left hand thread 74.

A chuck bolt 75 is threaded through the nut 73 and has an integral reduced stem 76 the end of which is provided with a male thread 77 for coupling with the internal thread 36 of insert D.

The process of installation of insert D involves the following steps:

(1) Insert D is threaded onto the tip 77 of chuck bolt 75 until its head 28 snugly engages the anvil head 71.

(2) With the hole 19 bored in the skin sheet 15, but with the sheet remaining flat and undimpled, the insert D is pushed through the hole 19 and the head 28 is forced into the sheet 15, d-impling it. This can be done by holding the anvil 70 in one hand and striking the head of the chuck bolt 75 a hammer blow (or it can be done by applying pressure in an arbor press).

(3) Wrenches are then applied to the flat faced shank 72 of the anvil and to the head of the bolt 75. The chuck bolt is turned in the direction indicated by the arrow 78 in FIG. 9, thus backing the chuck bolt out of the nut 73, retracting the stem 76 in the anvil 70, and thereby applying a pull to the body 27 of the insert while simultaneously applying reactive pressure of anvil head 71 against insert head 28. The resulting compression applied to the weakened section 62 causes the latter to bulge outwardly to produce the auxiliary head 61 which engages the edge of dimple 21 to lock the embedded flutes 30 in torqueanchorin'g engagement with the dimple.

An alternate method of installing the spacer of FIG. 3 may utilize the tool of FIG. 9 in slightly modified form, as illustrated in FIG. 11. In the modified tool, the anvil 70a is provided with a riveting head 54:: on the end of a reduced tubular neck 80 of the correct diameter to fit into the counterbore 46 of spacer B. The stem 7 6 of the chuck bolt 75 extends through the neck 80 and has its threaded tip 77 threaded fulllength through the body 27 of spacer sleeve C and threaded into an internally threaded Washer or nut 81 adapted to compressively engage the head 28 of plug C. Anvil 7041 has a radial annular shoulder 71a for compressive engagement against the head 25 of spacer sleeve B.

When the tool is first attached to the spacer assembly, the anvil 70a is retracted sufficiently to permit the insertion of the stem tip 77 through the plug C and into the nut 81. The tool is then operated in the manner described above in connection with insert D, applying a pull through the stem 76 to the plug C while simultaneously applying reactive pressure through the anvil shoulder 71a to the spacer head 25. The series of setting steps described above in connection with FIGS. 2, 2a and 2b then occurs, in the same order, the skin sheet 15 first being dimpled between head 28 and the end of sleeve body 23; the weakened section 6-1 of insert D then being collapsed and the head 25 dimpled into skin sheet 16; and finally, the riveting head 54a engaging the lip 29' of plug C and swaging it outwardly against spacer shoulder 31.

We claim:

1. A fastener for installation in a sandwich panel having spaced skin sheets joined by a core of low densty,

comprising: a tubular body including a relatively thick I main body portion, a weakened section defined by a counterbore in one end of said tubular body and joined to one end of said main body portion by an annular offset shoulder, and a preformed head on the end of said weakened section, projecting radially therefrom, and having a frusto-conical abutment shoulder engageable in a dimple in the outer face of one of said skin sheets around an aperture therein in which said fastener is receivable, said weakened section being adapted to be bulged outwardly under compression applied to said fastener, whereby to form a secondary head engageable with the inner face of said one skin sheet at the edge of said aperture in adjacent opposed relation to said preformed head so as to clamp said edge between said head and secondary head, said weakened section including a base portion gradually increasing in thickness toward said shoulder,

its inner wall being concavely curved and substantially tangent to the cylindrical wall of said counterbore and to said shoulder, on a radius substantially equal to the difference between the radii of said counterbore and of the interior of said thick-walled section, said weakened portion being of uniform thinness full length from its point of tangency with said base portion to a junction with said abutment shoulder, said base portion providing a buttressing characteristic effective to direct the outward- 1y bulging fold of said weakened section toward said skin sheet inner face under the effect of compression utilized for collapsing said weakened section and effectively clamping the dimple of said one skin sheet between said heads, with said secondary head leaning axially behind said dimple.

2. A sandwich panel and spacer type fastener assembly comprising: a sandwich panel including spaced skin sheets and a low density core joining said sheets, said panel having a hole in said core and an aperture in at least one of said skin sheets defining a mouth of said hole; a spacer sleeve including a relatvely thick main body portion, a weakened section joined to one end of said main body portion by an annular offset shoulder, and a head on the end of said weakened section, projecting radially therefrom, and engageable with the outer face of said one skin sheet around said aperture, and a secondary head constituted by an outwardly bulged weakened section of said tubular body adjacent said first mentioned head, in clamping engagement with the inner face of said one skin sheet; a plug having a bore to receive a fastener stud, having at one end a head provided w th a series of radiating flutes defining and under face theeof, said flutes being embedded in the outer surface of the other skin sheet, said other skin sheet having a marginal portion defining an aperture receiving sald plug, said marginal portion being clamped between said plug head and an opposed end of said spacer sleeve remote from said sleeve head, said plug being telescoped into said spacer sleeve and having means locking it thereto so as to resist pulling apart of said plug and sleeve.

3. A fastener-panel assembly as defined in claim 2, wherein said locking means comprises a reduced-thickness lip on the inner end of said plug expanded and engaged against said sleeve shoulder.

4. A fastener-panel assembly as defined in claim 2, wherein said plug has a through bore with an internal thread for coupling a threaded stud thereto.

5. A fastener-panel assembly as defined in claim 2, wherein said plug has a blind bore with an internal thread for coupling a threaded fastener stud thereto.

6. A fastener-panel assembly as defined in claim 2, wherein said plug has an unthreaded through bore,

7. A spacer-type fastener for installation in a hole in a sandwich panel having spaced skin sheets joined by a bridging core of low density, comprising: a plug including a tubular body having at one end a head provided with a series of radiating flutes defining an underface thereof for engagement with one of said skin sheets, and having at its other end a thinned lip defined by a counterbore therein; a spacer sleeve including a relatively thick main body portion in which said tubular plug body is snugly receivable, and, a weakened section joined to one end of said main body portion by an annular offset shoulder, and a head on the end of said weakened sectron, projecting radially therefrom, and engageable with the outer face of the other skin sheet around an aperture therein, said weakened section being adapted to be bulged outwardly under compression applied to said fastener, whereby to form a secondary head engageable with the inner face of said other skin sheet at the edge of said aperture so as to clamp said edge between said sleeve head and secondary head, the other end of said main body portion being engageable with the inner face of said one skin sheet around the aperture therein to clamp the same against said flutes of the plug head to provide torque-anchorage of the fastener to the panel, said plug lip being adapted to be expanded into engagement with said spacer sleeve shoulder to lock the sleeve and plug to one another so as to maintain said torque-anchorage, said plug head underface being a countersink'type, and wherein said other end of the spacer sleeve has a countersink for mating engagement with a dimple in said one skin sheet, clamped between said underface and said countersink.

8. A spacer-type fastener for installation in a hole in a sandwich panel having spaced skin sheets joined by a bridging core of low density, comprising: a plug including a tubular body having at one end a head provided with a series of radiating flutes defining and underface thereof for engagement with one of said skin sheets, and having at its other end a thinned lip defined by a counterbore therein; a spacer sleeve including a relatively thick main body portion in which said tubular plug body is snugly receivable, and, a weakened section joined to one end of said main body portion by an annular offset shoulder, and a head on the end of said weakened section, projecting radially therefrom, and engageable with the outer face of the other skin sheet around an aperture therein, said weakened section being adapted to be bulged outwardly under compression applied to said fastener, whereby to form a secondary head engageable with the inner face of said other skin sheet at the edge of said aperture so as to clamp said edge between said sleeve head and secondary head, the other end of said main body portion being engageable with the inner face of said one skin sheet around the aperture therein to clamp the same against said flutes of the plug head to provide torque-anchorage of the fastened to the panel, said plug lip being adapted to be expanded into engagement with said spacer sleeve shoulder to lock the sleeve and plug to one another so as to maintain said torqueanchorage spacer having a knurled cylindrical external surface on said weakened section thereof, said knurled surface providing increased column strength in said weakened section, such as to delay the formation of said secondary head while said dimple is being formed between said other end of the spacer and said plug head.

9. A fastener as defined in claim 8, wherein said increased column strength is such as to delay the formation of said secondary head while full clamping engagement of said one skin sheet between said plug head and said other end of the spacer is being developed.

10. A fastener as defined in claim 9, said knurling comprising a large number of closed spaced minute ridges in said external spacer surface, extending parallel to the spacer axis from said one end of the spacer to an area beyond said annular shoulder.

11. A fastener as defined in claim 8, said knur1- ing comprising a succession of ridges in said external spacer surface, extending parallel to the spacer axis, said ridges having a height in the range of approximately .OQ3.01O inch and a circumferential spacing between adjacent ridges in the range of approximately .0l0-.025 inch.

References Cited by the Examiner UNITED STATES PATENTS 2,409,352 10/1946 Gill -40 2,967,593 1/1961 Cushman 52-617 3,079,970 3/1963 Barry 29-521 X 3,131,743 5/1964 Hinkle l51-4l.74

RICHARD W. COOKE, JR., Primary Examiner. 

1. A FASTENER FOR INSTALLATION IN A SANDWICH PANEL HAVING SPACED SKIN SHEETS JOINED BY A CORE OF LOW DENSITY, COMPRISING: A TUBULAR BODY INCLUDING A RELATIVELY THICK MAIN BODY PORTION, A WEAKENED SECTION DEFINED BY A COUNTERBORE IN ONE END OF SAID TUBULAR BODY AND JOINED TO ONE END OF SAID MAIN BODY PORTION BY AN ANNULAR OFFSET SHOULDER, AND A PREFORMED HEAD ON THE END OF SAID WEAKENED SECTION, PROJECTING RADIALLY THEREFROM, AND HAVING A FRUSTO-CONICAL ABUTMENT SHOULDER ENGAGEABLE IN A DIMPLE IN THE OUTER FACE OF ONE OF SAID SKIN SHEETS AROUND AN APERTURE THEREIN IN WHICH SAID FASTENER IS RECEIVABLE, SAID WEAKENED SECTION BEING ADAPTED TO BE BULGED OUTWARDLY UNDER COMPRESSION APPLIED TO SAID FASTENER, WHEREBY TO FORM A SECONDARY HEAD ENGAGEABLE WITH THE INNER FACE OF SAID ONE SKIN SHEET AT THE EDGE OF SAID APERTURE IN ADJACENT OPPOSED RELATION TO SAID PREFORMED HEAD SO AS TO CLAMP SAID EDGE BETWEEN SAID HEAD AND SECONDARY HEAD, SAID WEAKENED SECTION INCLUDING A BASE PORTION GRADUALLY INCREASING IN THICKNESS TOWARD SAID SHOULDER ITS INNER WALL BEING CONCAVELY CURVED AND SUBSTANTIALLY TANGENT TO THE CYLINDRICAL WALL OF SAID COUNTERBORE AND TO SAID SHOULDER, ON A RADIUS SUBTANTIALLY EQUAL TO THE DIFFERENCE BETWEEN THE RADII OF SAID COUNTERBORE AND OF THE INTERIOR OF SAID THICK-WALLED SECTION, SAID WEAKENED PORTION BEING OF UNIFORM THINNESS FULL LENGTH FROM ITS POINT OF TANGENCY WITH SAID BASE PORTION TO A JUNCTION WITH SAID ABUTMENT SHOULDER, SAID BASE PORTION PROVIDING A BUTTRESSING CHARACTERISTIC EFFECTIVE TO DIRECT THE OUTWARDLY BULGING FOLD OF SAID WEAKENED SECTION TOWARD SAID SKIN SHEET INNER FACE UNDER THE EFFECT OF COMPRESSION UTILIZED FOR COLLAPSING SAID WEAKENED SECTION AND EFFECTIVELY CLAMPING THE DIMPLE OF SAID ONE SKIN SHEET BETWEEN SAID HEADS, WITH SAID SECONDARY HEAD LEANING AXIALLY BEHIND SAID DIMPLE. 